Extension cords usually have multiple wire conductors, each of which has its own insulator, which are then contained within an outer jacket covering. Cords with multiple conductors may also be bundled together with tape or a binder prior to jacketing.
Extension cords are sold coiled and uncoiled. Uncoiled extension cords are good for short distances, and can be stored in a retracting wheel device when they are longer. Coiled extension cords rely on their coiling to keep them neat when not extended, and they typically have a coiled diameter of one and one half to two times the cord diameter. In this regard, although it is not an extension cord, telephones commonly have coiled cords.